Accurate cardiac event detection in an implantable cardiac stimulus device

ABSTRACT

Methods, systems, and devices for signal analysis in an implanted cardiac monitoring and treatment device such as an implantable cardioverter defibrillator. In some illustrative examples, detected events are analyzed to identify changes in detected event amplitudes. When detected event amplitudes are dissimilar from one another, a first set of detection parameters may be invoked, and, when detected event amplitudes are similar to one another, a second set of detection parameters may be invoked. Additional methods determine whether the calculated heart rate is “high” or “low,” and then may select a third set of detection parameters for use when the calculated heart rate is high.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/399,901, filed Mar. 6, 2009, now U.S. Pat. No. 8,565,878, whichclaims the benefit of and priority to U.S. Provisional PatentApplication Ser. No. 61/034,938, filed Mar. 7, 2008 and titled ACCURATECARDIAC EVENT DETECTION IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE, thedisclosure of which is incorporated herein by reference.

The present application is related to U.S. patent application Ser. No.12/399,914, filed Mar. 6, 2009 and titled METHODS AND DEVICES FORIDENTIFYING AND CORRECTING OVERDETECTION OF CARDIAC EVENTS, published asUS Patent Application Publication Number 2009-0259271, now U.S. Pat. No.8,160,686, which claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/051,332, filed May 7, 2008, and thedisclosures of which is also incorporated herein by reference.

FIELD

The present invention relates generally to implantable medical devicesystems that sense and analyze cardiac signals. More particularly, thepresent invention relates to implantable medical devices that capturecardiac signals within a patient's body in order to classify cardiacactivity and direct therapy for treatment of arrhythmias.

BACKGROUND

Implantable cardiac stimulus devices typically sense cardiac electricalsignals within a patient in order to classify the patient's cardiacrhythm as normal/benign or malignant in order to prevent, treat, orterminate malignant rhythms. Such malignant rhythms can include, forexample, ventricular fibrillation and some ventricular tachycardias. Howaccurately an implantable medical device analyzes captured signalsdetermines how appropriately it can direct therapy.

New and alternative methods and devices for detection and/or analysis ofcaptured cardiac events in implantable medical devices are needed.

SUMMARY

Various illustrative embodiments of the present invention are directedtoward improving accuracy of cardiac event detection by implantablemedical devices. The invention may be embodied in methods and/ordevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing a method for detection includingidentifying and responding to amplitude similarity/dissimilarity ofdetected events;

FIG. 2 illustrates a representative subcutaneous implantable cardiacstimulus system;

FIG. 3 illustrates a prior art detection profile;

FIG. 4 graphically depicts overdetection of a captured cardiac signal;

FIG. 5 illustrates calculation of “estimated peak” using an average oftwo prior peaks;

FIGS. 6A-6B show illustrative detection profiles;

FIGS. 7A-7B graphically show changes in dynamic detection profiles basedon similarity/dissimilarity measures of captured signals;

FIG. 8 is a flow diagram of an illustrative example of cardiac signalanalysis in an implantable medical device;

FIG. 9 is a flow diagram of another illustrative example of cardiacsignal analysis in an implantable medical device;

FIG. 10 illustrates detection using an illustrative example of detectionprofiles during onset of ventricular fibrillation;

FIG. 11 illustrates a set of detection profiles and parameters for anillustrative example; and

FIG. 12 illustrates a full set of detection profiles and parameters foranother illustrative example.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

Unless implicitly required or explicitly stated, the methods below donot require any particular order of steps. It should be understood thatwhen the following examples refer to a “current event,” in someembodiments, this means that the most recently detected event is beinganalyzed. However, this need not be the case, and some embodimentsperform analysis that is delayed by one or more event detections or afixed period of time.

Implantable devices typically calculate a heart rate or beat rate forthe implantee. Heart or beat rate is typically given in beats-per-minute(bpm). Such devices then use the heart rate either alone or inconjunction with some other factor (sometimes morphology is used, forexample) to determine whether the implantee needs therapy.

The calculation of heart rate can be performed by observing the rate atwhich “events” are detected by the implanted device. In an illustrativeexample, an event is detected by comparing received signals to adetection threshold, which is defined by a detection profile.Illustrative examples of detection profiles are shown in FIGS. 3, 6A, 6Band 11-12. A detected event is declared when the received signal crossesthe detection threshold.

A cardiac electrogram includes several portions (often referenced as“waves”) that, according to well known convention, are labeled withletters including P, Q, R, S, and T, each of which corresponds toparticular physiological events. It is typical to design detectionalgorithms to sense the R-wave, though any portion, if repeatedlydetected, can be used to generate a beat rate. If morphology (shape)analysis is used in addition to heart rate, the system may captureand/or analyze the portion of the cycle that includes the Q, R and Swaves, referred to as the QRS complex. Other portions of the patient'scardiac cycle, such as the P-wave and T-wave, are often treated asartifacts that are not sought for the purpose of estimating heart rate,though this need not be the case.

Sensing may be performed in the near field or far field. Intracardiacelectrograms are dominated by signal components generated in the nearfield, while surface or subcutaneous sensing captures signals in the farfield. The R-wave often has larger amplitude than other portions of thecardiac cycle, though this can vary depending upon how and from whatlocation the signal is sensed and/or with patient physiology.

Typically, for purposes of ascertaining rate each cardiac cycle iscounted only once. Overdetection (such as a double or triple detection)may occur if the device declares more than one detected event within asingle cardiac cycle. This may happen if an R-wave and a trailing T-waveare both detected from a single cardiac cycle or if a wide QRS complexis detected twice. Overdetection may also occur if noise causes an eventto be declared when no cardiac event has taken place, for example, dueto external noise, pacing artifact, skeletal muscle noise,electro-therapy, etc.

Overdetection can lead to overcounting of cardiac cycles. For example,if one cardiac cycle takes place and a detection algorithm declaresmultiple detected events, overdetection has occurred. If the heart rateis then calculated by counting each of these detections, overcountingoccurs.

Calculated heart rates may be used alone or in combination with otherfactors to classify cardiac activity as malignant or benign. Therapydecisions are usually made based upon such classification. Overcountingin reliance on overdetected events can result in erroneously high ratecalculation. Miscalculation of heart rate can lead to incorrect therapydecisions and, particularly, incorrect therapy delivery. However, simplypreventing overdetection by rendering a device insensitive to receivedsignals can cause undersensing, which may impair or delay delivery ofneeded therapy.

An illustrative embodiment makes use of a detection method as shown inthe high-level functional block diagram of FIG. 1. The method is brieflyintroduced here, with more detailed examples provided below. Theillustrative method makes use of a detection profile as shown in one ofFIGS. 3, 6A, 6B, 11 and/or 12. In the illustrative example of FIG. 1,when detected events are similar to one another, a relatively moresensitive detection profile is used, and when detected events aredissimilar from one another, a relatively less sensitive detectionprofile is used.

As shown at step 10, a peak for a recent detected event is compared to aprior peak. The illustrative example uses the comparison at 10 tocategorize the recent detected event peak as either similar 12 ordissimilar 14 relative to the prior peak. The comparison at 10 may takethe following form, for example:

$A \leq \frac{{New}\mspace{14mu}{Peak}\mspace{14mu}{Amplitude}}{{Prior}\mspace{14mu}{Peak}\mspace{14mu}{Amplitude}} \leq B$where A and B are predetermined values. In the illustrative example, ifthe above formula yields a “True” outcome, then the peaks are similar;otherwise, they are dissimilar.

The quotient in the middle of this formula is referred to as the peakratio. In an illustrative example, A=0.8 and B=1.2. In other examples, Amay be in the range of 0.5-0.9, and B may be in the range of 1.1-1.5.Additional examples of similar/dissimilar analysis are provided below.

If the recent detected event peak is similar to the prior peak, as shownat 12, a “Similar” detection profile is applied, as shown at 16. On theother hand, if the recent detected event peak is dissimilar from theprior peak, as shown at 14, a “Dissimilar” detection profile is applied,as shown at 18. The selection of the Similar or Dissimilar detectionprofile modifies the sensitivity of the detection method. In oneexample, the Similar Detection profile is more sensitive than theDissimilar Detection profile as shown at 20A/20B. In another example,the Similar Detection profile is less sensitive than the DissimilarDetection profile, as shown at 22A/22B.

The adopted Similar or Dissimilar detection profile 16, 18 is then usedto detect the next detection profile threshold crossing, as shown at 24.The method then iterates through A 26.

Examples where a detection profile is more or less sensitive are shownbelow. In short, a detection profile typically defines amplitudes atgiven points in time, and if the captured signal exceeds the detectionprofile defined amplitude, a detection occurs. By raising or loweringthe detection profile and/or modifying the timeline of the detectionprofile, sensitivity is raised or lowered.

In another embodiment, the similar/dissimilar analysis may include aninterval rule. For example, the likelihood of double detection decreaseswhen the interval between two detections is long. In an illustrativeembodiment, two consecutive detections separated by a relatively longinterval (greater than, for example, 500 milliseconds) are not subjectto the similar/dissimilar analysis, as they are likely not overdetectedduring the long interval. Instead, when an interval of a length greaterthan a predetermined threshold is identified, one or the other of thesimilar or dissimilar detection profile is adopted automatically.

It is contemplated that the present invention may be embodied in severalforms including at least implantable cardiac monitoring systems andimplantable cardiac stimulus systems. An illustrative subcutaneouscardiac stimulus system is shown in FIG. 2. The subcutaneous system isshown relative to a heart 30, and includes a canister 32 coupled to alead 36. The canister 32 houses operational circuitry for performinganalysis of cardiac activity and for providing a stimulus output. A canelectrode 34 is disposed on the canister 32. In some embodiments, ratherthan a discrete electrode 34, a surface of the canister 32 may serve asan electrode.

The lead 36 includes three illustrative electrodes shown as ringelectrode 38, coil electrode 42, and tip electrode 40. These electrodes38, 40, 42 and the can electrode 34 may define a plurality of sensingvectors, such as V1, V2, V3 and, optionally, V4. If desired, one or morevectors V1, V2, V3, and V4 may be chosen for use as a default sensingvector, for example, as discussed in US Patent Application PublicationNumber 2007-0276445 titled SYSTEMS AND METHODS FOR SENSING VECTORSELECTION IN AN IMPLANTABLE MEDICAL DEVICE. Illustrative subcutaneoussystems are also shown in U.S. Pat. Nos. 6,647,292 and 6,721,597, and7,149,575. Stimulus may be applied using any chosen pair of electrodes;one illustrative example uses the can electrode 34 and the coilelectrode 42 to deliver stimulus. In yet another embodiment, multiplesensing vectors may be used simultaneously.

A programmer 44 is also shown. The programmer can be used to configurethe implant system as desired through methods that are widely known.These may include, for example, radiofrequency or inductive telemetrycommunication.

The present invention is not limited to any particular hardware, implantlocation or configuration. Instead, it is intended as an improvementupon any implantable cardiac monitoring and/or treatment system.Embodiments of the present invention may take the form of devices orsystems for use as subcutaneous-only, transvenous single ormulti-chamber, epicardial or intravascular implantable defibrillator ormonitoring systems, or as methods of use in any such system.

FIG. 2 omits various anatomical landmarks. The illustrative system shownwould be implanted outside of the ribcage (not shown) of the implantee.The location illustratively shown places the canister 32 atapproximately the left axilla of the implantee, level with the cardiacapex, with the lead 36 extending medially toward the sternum and thentoward the head of the patient along the left side of the sternum. Forexample, the implant may be similar to that shown in commonly assignedUS Patent Application Publication Number 2006-0122676 titled APPARATUSAND METHOD FOR SUBCUTANEOUS ELECTRODE INSERTION, now U.S. Pat. No.7,655,014.

The canister 32 may be placed in anterior, lateral, and/or posteriorpositions including, without limitation, axillary, pectoral, andsub-pectoral positions, as well as placements on either the left orright side of the patient's torso. The lead 36 may then be placed in anyof a number of suitable configurations including anterior-posteriorcombinations, anterior-only combinations, transvenous placement, orother vascular placements. An embodiment of a monitoring system may be asubcutaneously implanted system having a housing with multipleelectrodes thereon, with or without a lead.

FIGS. 3-5 provide an introduction to a detection profile and its use.The application of the detection profile of FIG. 3 to a captured cardiacsignal is shown in FIG. 4, which illustrates overdetection using such aprofile. FIG. 5 illustrates calculation of “estimated peak” that is usedto generate amplitudes defined by a detection profile as shown in FIGS.3-4. It should be noted that, for purposes of simplicity, the detectionprofiles shown herein are illustrated using a rectified signal. Those ofskill in the art will recognize that the detection profile for anunrectified signal would effectively define the detection profile on thenegative side of the sensing baseline as well.

FIG. 3 illustrates a detection profile at 50, with portions thereoflabeled for illustrative purposes. The detection profile includes arefractory period, shown in cross-hatching. The refractory period is aninitial time period that follows a threshold crossing. During therefractory period, captured signal data may be recorded and/or analyzed,but additional detected events are not declared. Following therefractory period is an exponential decay period, as shown. Theexponential decay brings the detection threshold down, over time, from astarting point to the sensing floor of the device. Some challenges withthis detection profile are discussed by U.S. Pat. No. 5,709,215 toPerttu et al.

The “sensing floor” may be defined by the hardware limits of the deviceand/or by the ambient noise environment of the device. A sensing floormay also be selected in any suitable manner. Values for the sensingfloor may vary depending upon the characteristics of the particularimplantable cardiac stimulus system including, for example, inputcircuitry, filter capability, electrode location and size, and patientphysiology.

As used herein, and for illustration purposes, the height shown for thedetection profile during each refractory period represents the“estimated peak” amplitude of the cardiac signal at that time. Inoperation, the implanted device makes use of one or more prior detectedevents to estimate the amplitude of peaks in the cardiac signal.Illustrative calculations of estimated peak are shown in FIG. 5. In theillustrative detection profile of FIG. 3, the exponential decayfollowing the refractory period uses the estimated peak as its startingpoint, and follows an exponential decay curve from the estimated peak tothe sensing floor or some other selected value.

FIG. 4 illustrates a problem that may arise during application of thedetection threshold of FIG. 3, which is shown at 64, to a capturedcardiac signal 62. In FIG. 4, refractory periods are indicated bycross-hatching, as shown at 60, 66, 68, 70 and 72. Refractory periods at60, 66 and 70 cover QRS complexes in the captured signal 62; thesedetections can be considered “accurate,” as the desired portion of thecardiac signal has been detected.

T-waves are shown at 74, 76 and 78. As can be seen at 74, the T-wavefollowing refractory period 60 does not cause a detection, although itis close in amplitude to the decaying detection profile 64. The nextT-wave, at 76, crosses the decaying detection profile, resulting in adetection followed by refractory period 68. The detection of the T-wave76 creates two potential problems. First, an overdetection occurs, sincetwo detections (resulting in refractory periods 66, 68) occur in asingle cardiac cycle. Second, the T-wave 76 has a different amplitudethan the R-waves of the captured signal and can therefore affect thecalculation of estimated peak, as shown by FIG. 5.

Referring to FIG. 5, the illustrative example uses the average amplitudeof two prior peaks as the “estimated peak.” As shown at 80, correctidentification of QRS complexes enables a calculation of estimated peakthat is an average of the R-wave amplitude for the previous two QRScomplexes. As shown at 82, however, detection of the T-wave as thesecond peak causes a calculation of estimated peak that may be lowerthan the R-wave peak.

Returning to FIG. 4, the estimated peak shown at 68 is an average of theamplitudes for R-waves R1 and R2, however, the estimated peak shown at70 is an average of the amplitudes for R-wave R2 and T-wave T2. Sincethe T-waves are lower in amplitude than the R-waves, as shown at 70, theestimated peak following T-wave 76 is lowered, increasing the likelihoodthat another T-wave will also cause a threshold crossing and detection.In the illustrative example, T-wave 78 crosses the detection threshold,causing the system to again declare a detected event. Thus T-wave 76contributes to the overdetection of T-wave 78, and the overdetection ofT-waves becomes a self-perpetuating condition.

FIGS. 6A-6B show illustrative detection profiles that can be manipulatedin accordance with some examples of the present invention. Referring toFIG. 6A, a detection profile is shown at 90 and includes a refractorysegment having a refractory duration 92, which is immediately followedby a first constant threshold segment (CT1) using a CT1% of theestimated peak for its amplitude and a CT1 duration 94. Following CT1 isa second constant threshold segment (CT2) using a CT2% of the estimatedpeak for its amplitude and CT2 duration 96. Following CT2 is anexponential decay which begins at amplitude CT2% of the estimated peakand decays toward the sensing floor.

For the illustrative example of FIG. 6A, at least the followingvariables may be manipulated to change the sensitivity of the detectionprofile:

Durations 92, 94, or 96;

Amplitudes CT1%, CT2% of the estimated peak;

The start point of the Exponential Decay; and/or

The time constant of decay for the Exponential Decay.

In illustrative examples, these variables are manipulated singly or incombination to increase or decrease sensitivity in response toidentified similarity or dissimilarity between detected event peakamplitudes. For example, extending any of the durations 92, 94, 96reduces the sensitivity of the overall detection profile. In someembodiments, the refractory period 92 remains fixed, while combinationsof the other variables are modified.

FIG. 6B illustrates another detection profile 100. FIG. 6B incorporatesa “dynamic floor.” The dynamic floor is a detection profile componentthat is set to a selected value above the sensing floor and used as anintermediate “floor” for the detection profile. An illustrative dynamicfloor is invoked until a dynamic floor time-out (DFTO), at which timethe detection profile begins decaying toward the sensing floor.

Referring again to FIG. 6B, a detection profile 100 includes arefractory segment having a refractory duration 102, which is followedby a first constant threshold segment (CT 1) using CT1% of the estimatedpeak as its amplitude and having a CT1 duration 104. After CT1 is asecond constant threshold segment (CT2) using CT2% of the estimated peakas amplitude, and having a CT2 duration 106. Next is a first decayperiod, which starts from the amplitude of CT2% and ends at a dynamicfloor having an amplitude DF %, with each of CT2% and DF % based on theestimated peak. The DFTO 108 is used to define the duration of the firstdecay. Following the first decay is a second decay to the sensing floor.The first and second decays may use the same time constant of decay, ormay use different time constants of decay.

For the example shown in FIG. 6B, the inclusion of the dynamic floor anda DFTO 108 provides two additional variables that can be modified inresponse to identified similarity/dissimilarity. While not shown, in yetanother embodiment, CT2 may be omitted such that the first decay startsfrom CT1%, or some other predetermined percentage of estimated peak, oreven from a constant not associated with the estimated peak. In anotherexample, CT2 is used as a placeholder for the start of the first decayperiod and is given a very short duration equal to a single sampleperiod. While exponential decays are shown in FIGS. 6A-6B, any suitabledecay shape may be used, for example, including constant ramp decays orother non-exponential functions, for example.

FIGS. 7A-7B show illustrative adaptive profiles and a system-levelresponse to changes in peak amplitudes. In FIG. 7A, a first detection isshown at 120. The detection profile is shown in a form generallycorresponding to that of FIG. 6B, though a detection profile as in FIG.6A could also be used. Additional detections occur at 122, 124 and 126.

In the illustrative example of FIG. 7A, immediately prior to detection120, there were consecutive similar peaks (not shown). This leads to theinclusion of a relatively short CT1 and low CT1%, as indicated. Withthese parameters, as shown at 130, a T-wave nearly creates a detectionthreshold crossing.

The illustrative system keeps track of the peak amplitude during therefractory periods (again shown as cross-hatched blocks). The peakvalues are shown beneath the refractory periods in analog-to-digitalconversion (ADC) units. ADC units represent the output ofanalog-to-digital conversion within the device; in the Figures theseunits are shown merely to help illustrate other concepts.

The peak values are used to calculate peak ratios shown at 128. The peakratio for detections 120 and 122 is 0.92. In this illustrative example,peak ratios of about 0.8-1.2 are defined as indicating “similar” peaks,so detections 120 and 122 are considered similar. Other ranges definingsimilar/dissimilar peak ratios, and other measures of similar/dissimilarmay be used.

The detection profile following detection 122 is similar to thedetection profile following detection 120 because the prior peakamplitude for detection 120 is similar to that of the immediatelypreceding peak amplitude (the preceding peak is not shown). T-wave 132following detection 122 causes an overdetection 124. The peak for T-wave132 is lower than the R-wave peak for detection 122. These peaks yield apeak ratio of 0.70, which falls outside of the range that defines“similar” peak ratios for the example (0.8-1.2 being considered“similar”).

The system as shown has a built-in delay of one event, so detectionfollowing the overdetection 124 uses the “similar” detection profile.However, in contrast to events 120, 122 and 124, the detected event at126 is followed by a detection profile based on “dissimilar” detectionprofile parameters. This results in modifications, as indicated,including extended CT1 duration and a higher CT1%. As a result, theT-wave shown at 134 does not cause a detection threshold crossing and nodetected event is declared for T-wave 134. The modification in view ofthe dissimilar peak amplitudes prevents continued overdetection in theillustrative example shown in FIG. 7A.

In an illustrative example, FIG. 7A makes use of the following detectionprofile parameters (% indicates percent of the estimated peak):

Dissimilar Similar Refractory: 200 ms 200 ms CT1%: 95% 80% CT1 Duration:350 ms 200 ms CT2%: 50% 50% CT2 Duration:  4 ms  4 ms DF %: 50% 37.5%  DFTO: 720 ms from the start of Refractory

The inclusion of DF % and DF TO is not apparent from FIG. 7A and, ifdesired, these may be omitted in some embodiments. The time constantsfor decay may be any suitable value. In an illustrative example, thetime constant of decay for the above parameters is in the range of 400milliseconds. Additional variations of and ranges for these parametersare provided below.

FIG. 7B shows the analysis as it continues, with more detected eventsshown. Starting at the left, detected event 150 is associated with adetection profile using a “similar” detection profile configuration.This results in overdetection of the trailing T-wave, shown at detection152. As shown following the next detection, at 154, detection 152 of aT-wave results in reduction of the estimated peak (again, the estimatedpeak is shown as the height of the cross-hatched block that representsthe refractory period).

However, the overdetection 152, considered relative to peak 150, resultsin calculation of a peak ratio of 0.63 (peak ratios are shown at 162).As indicated by the line/arrow 164, a low peak ratio causes the use of a“dissimilar” detection profile configuration following the detection at154. The delay in this illustrative example is based on a hardwareenvironment in which the peak associated with a given refractory periodis not read as a peak until after the end of the given refractoryperiod. It is contemplated that in some hardware environments, the peakand peak ratio could be found in real-time, such that a one beat delayis avoided. In such an example, a “dissimilar” configuration could beinvoked during or following the refractory period of detection 152.

Once the “dissimilar” configuration is invoked following detection 154,the detection profile successfully passes over the next T-wave 160. Thenext detection, shown at 156, is again an accurate detection caused byan R-wave. Because the T-wave detection at 152 is dissimilar in heightfrom the detection at 154 (as well as 150), the peak ratio of 1.57causes the continued use of the “dissimilar” configuration followingdetection 156. Again, the detection profile successfully passes over aT-wave. Detection 158 follows. As indicated by the line/arrow 166, thesimilarity of the peaks for detections 154 and 156 (peak ratio of 1.01)causes the resumption of the more sensitive “similar” configuration.

As shown at 168, the T-wave following detection 158 is detected. The“dissimilar” detection profile configuration will be invoked again. Asshown in this illustrative example, during time periods in whichoverdetection is avoided, similar peaks occur and the more sensitivedetection profile configuration associated with similar peaks isinvoked. Thus a cycle can develop in which the device transitionsbetween dissimilar and similar detection profile configurations.

The illustrative detection pattern results in sets of four detections inwhich three R-waves and one overdetected T-wave appear. If the actualheart rate is 100 bpm, consistent overdetection of every T-wave (forexample, as shown in FIG. 4) would yield a calculated rate of 200 bpm. Abeat rate of 200 bpm may be considered tachyarrhythmic for a substantialnumber of patients who are candidates for ICD implantation and maycreate a risk of inappropriate therapy. The example of FIG. 7B, however,would calculate a rate of about 133 bpm, which is unlikely to causeinappropriate therapy.

If desired, a counter or other hysteresis tool may be used to slow thecycling between “similar” and “dissimilar” detection profileconfigurations. In an illustrative example, once invoked, a detectionprofile configuration would be used for some predetermined number ofdetections before invoking a different detection profile configuration.For example, at least 5 detections would occur using a detection profileconfiguration before a different one could be called. In anotherexample, the hysteresis could be “one-sided,” that is, hysteresis couldapply only when one of the “similar” or “dissimilar” configurations isinvoked. In the example of FIG. 7B, no added hysteresis is provided toavoid delayed identification of malignant fast arrhythmias such asventricular fibrillation.

FIG. 8 is a flow diagram of an illustrative example of cardiac signalanalysis in an implantable medical device. The illustrative example ofFIG. 8 includes a detection loop shown at 200, in which incoming signalis filtered, amplified and sampled, as shown at 202. The signal may berectified in block 202, if desired.

The samples are then compared to a threshold defined by a detectionprofile as indicated at step 204. Once a threshold crossing occurs, thedetection loop 200 is exited and a detected event is declared as shownat 206. If morphology analysis is used, when the detected event isdeclared 206, various steps may be taken to define a sample windowassociated with the detected event, for example, as discussed incommonly assigned US Patent Application Publication Number 2006-0116595,now U.S. Pat. No. 7,376,458 and titled METHOD FOR DEFINING SIGNALTEMPLATES IN IMPLANTABLE CARDIAC DEVICES; and/or commonly assigned USPatent Application Publication Number 2006-0116725, now U.S. Pat. No.7,477,935 and titled METHOD AND APPARATUS FOR BEAT ALIGNMENT ANDCOMPARISON.

Next, preliminary analysis is performed, as indicated at 208. This mayinclude, for example, waveform appraisal discussed in commonly assignedU.S. Pat. No. 7,248,921, titled METHOD AND DEVICES FOR PERFORMINGCARDIAC WAVEFORM APPRAISAL. If the preliminary analysis 208 reveals thatthe detected event 206 does not appear to be a cardiac event (or acardiac event masked/covered by substantial noise), the detected event206 is identified as a suspect event, and data associated with thedetected event 206 is discarded, with the method then reverting back tothe detection loop 200 using the same detection parameters that werepreviously in use. In some embodiments, step 208 may be omitted.

If preliminary analysis 208 is passed, then rhythm analysis isperformed, as indicated at 210. Rhythm analysis may include any of anumber of steps/methods. One illustrative example uses calculated heartrates and/or morphology analysis to create detected event markers thatindicate whether a given detected event is “shockable” or“nonshockable”. Morphology analysis may include, for example, comparisonto a stored or dynamically changing template (for example, usingcorrelation waveform analysis), QRS width analysis, and/or othershape-based analysis.

A buffer of shockable/nonshockable markers may be maintained as an X/Ycounter. If a predetermined X/Y ratio is met, then the X/Y counterindicates therapy. For example, an 18/24 threshold may be used, where,if 18 of the previous 24 detected events that pass preliminary analysisare “shockable,” the X/Y counter indicates therapy. The phrase“indicates therapy” is intended to mean that the implanted device hasidentified a treatable condition and therefore indicates that therapy islikely needed by the patient.

In addition, one or more persistence factors may be considered.Persistence may be observed by determining whether the X/Y counterindicates therapy for a threshold number of consecutive detected events.Illustrative examples of persistence analysis are set forth in commonlyassigned US Patent Application Publication Number 2006-0167503, now U.S.Pat. No. 8,160,697, titled METHOD FOR ADAPTING CHARGE INITIATION FOR ANIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR. For example, the persistencefactor (if included) may call for the X/Y counter to indicate therapyfor a minimum number, N (the persistence factor), of consecutiveiterations. Where non-sustained tachycardias are identified, thepersistence factor may be incremented to avoid shocking a non-sustainedrhythm. In one example, N=2 initially and is increased by steps of 3if/when nonsustained tachycardia occurs.

These methods are illustrative and no particular step is required toperform rhythm analysis 210.

Unless detection is suspended (for example, detection may be suspendedduring and shortly after a stimulus delivery or by a physician duringtelemetric communications with an implant), the method also performssteps to prepare for return to the detection loop 200. These steps mayinclude determination of whether similar or dissimilar detected eventpeaks are observed, as shown at 212. The outcome of analysis at step 212determines the detection profile configuration used to set the detectionprofile in step 214. The detection profile, as configured in step 214,is then used upon return to the detection loop 200.

The above examples of “similar” and “dissimilar” detection profileconfigurations may be used in step 214 to modify the detection profile.As shown by the examples of FIG. 7A, step 214 may reduce the likelihoodof persistent overdetection. This may, in turn, increase the accuracy ofrhythm classification. As indicated by FIG. 7B, overdetection may occureven with step 214, however, because the modifications can reduce thefrequency with which overdetections occur, the method helps avoidincorrect therapy decisions.

FIG. 9 is a flow diagram of another illustrative example of cardiacsignal analysis in an implantable medical device. A detection loop 250is shown again including steps of filtering, amplifying and sampling 252and comparing the captured signal to a threshold 254. A thresholdcrossing causes an exit from the detection loop 250, and a detectedevent is declared as shown at 256. Preliminary analysis 258 is againperformed, as before, with noisy or suspect event identification causinga return to the detection loop 250.

If the preliminary analysis block 258 is passed, the method continues bycalculating a heart rate, as indicated at 260. In some illustrativeexamples, double detection analysis may be performed prior tocalculating heart rate, as noted at 262. Block 262 may be omitted, ifdesired.

Returning to step 260, if the heart rate is relatively high, the methodcontinues by performing additional double detection analysis as shown at264. The double detection analysis at 264 may be different and inaddition to the analysis noted at 262. Double detection analysis 262,264 may include any suitable method for identifying double detections.Some examples can be found in U.S. Provisional Patent Application No.61/051,332, titled METHODS AND DEVICES FOR IDENTIFYING AND CORRECTINGOVERDETECTION OF CARDIAC EVENTS. The following are examples of analysesthat may occur in blocks 262 or 264:

-   -   Identify High-Low-High correlation pattern(s) for detected        events and correlation template(s).    -   Identify double detections resulting from multiple detections of        wide cardiac complexes, which may include observation of whether        pairs of detected events are very close together in time and        have certain shape characteristics.    -   Identify Long-Short-Long interval pattern between detected        events.

Other factors for identification of double detection (or of otheroverdetection such as triple detection) may instead be used, if desired,in step 262, 264.

Following block 264, rhythm analysis is performed as indicated at 266.Next, the detection profile is set, as shown at 268, using yet anotherdetection profile configuration, this one being used when the calculatedrate (step 260) is relatively high (FIGS. 11-12 show examples). Themethod then iterates at A 270.

If the heart rate is relatively low, the analysis determines whetherdetected event peaks are similar or dissimilar, as shown at 272. Themethod sets the detection profile using similarity/dissimilarityinformation, as shown at 274. The method next returns to the detectionloop 250 via block A 270.

In an illustrative example, the “relatively low” and “relatively high”rates are calculated on the basis of an average of intervals betweendetected events that pass preliminary analysis 258 and, if included, thefirst pass of double detection analysis at 262.

In an illustrative example, the calculated rate is Low when a heart rateof less than about 148 bpm is calculated from an average of fourintervals between detected events. Further in the illustrative example,the calculated rate is High when a heart rate greater than about 167 bpmis calculated. In the example, these values may lie on either side of ahysteresis band in which the characterization of “High” and “Low” ratesdepends upon the characterization of rate for the previous detectedevent. Thus, in the illustrative example, if the newly calculated rateis 155 bpm, and the previous rate was characterized as “High,” then therate is again characterized as “High”. Other definitions of “High” and“Low” may be used and/or these values may be programmed during atelemetry session.

In the illustrative example, the modifications to detection profile inview of similarity and dissimilarity are not used at step 268 for highrates. FIG. 10 provides an example showing how invocation of a“dissimilar” detection profile can slow the identification of aventricular fibrillation. FIG. 11 provides illustrative examples of setsof parameters that may be used in the embodiment of FIG. 9 to definedetection profiles. The addition of the fast detection profileparameters may avoid a delay in identification of a malignant rhythmshown in FIG. 10. FIG. 12 provides an even more detailed example.

The illustrative method of FIG. 9 uses computationally expensive“enhanced analysis” (at blocks 264 and 266) when calculated rates arerelatively high, and simpler computations when calculated rates arerelatively low. One example has been discussed above relative to FIG.7B: a cardiac rhythm having an intrinsic rate of 100 cardiac cycles perminute included relatively large T-waves that caused overdetection. Thecyclic invocation of the “dissimilar” detection profile configurationpasses over the majority of the T-waves, resulting in a calculation of133 bpm, rather than 200 bpm that would occur if each T-wave wascounted. In the example, the peak ratio calculation allows use ofdetection profile modifications to reduce reliance on morecomputationally costly tools, potentially reducing power consumption.

FIG. 10 illustrates detection using an illustrative example of detectionprofiles during onset of ventricular fibrillation. A detection is shownat 300 with an R-wave peak occurring during the refractory period, andthe detection profile easily passes over the following T-wave. Forillustrative purposes, the detection profile following detection 300 isdefined using “similar” peak parameters.

As shown at 302, the cardiac rhythm devolves into a ventricularfibrillation (VF), characterized by fast-moving, low amplitude peaks.The first detection of a VF peak occurs at 304. The amplitude fordetection 304 is reduced significantly relative to peak 300. The systemcontinues to use the same “similar” peak parameters based uponcomparison of the detection at 300 to a prior peak. Another peak isdetected at 306, with underdetection or “dropout” occurring for severalVF Peaks. As indicated, based on the difference between peaks atdetection 300 and detection 304, “dissimilar” parameters are usedfollowing detection 306. Due to averaging of the two prior detections,the estimated peak for detection 306 is still quite a bit higher thanthe current peak.

Continuing across FIG. 10, it can be seen that a VF peak is captured at308, and is followed at 312 with another detected VF peak. In essence,the estimated peak measurement allows the detection profile to chasedown the VF signal by lowering the amplitude of the highest portions ofthe detection profile as lower amplitude signals are captured.

VF is often inconsistent in amplitude and the baseline may wander. As aresult, the use of the “dissimilar” profile may create additionalproblems with detection due to the intrinsic variability of VF. Forexample, the peak for detection 312 is higher than the peak fordetection 308. As shown at 314, the variability yields peak ratiosindicating that the peaks are “dissimilar.” The dissimilar profile isthen invoked as shown at 316. Because the dissimilar profile in thisillustrative example is relatively less sensitive than the similarprofile, this can delay further detections, as shown. One or morecardiac cycles can then go undetected by the implanted device. Moreunderdetection may follow, since, as shown at 314, peak 312 isdissimilar from the following peak.

The upper portion of the Figure indicates detection intervals, at 320.As can be seen at 322, a relatively long interval is created by thedissimilar detection profile configuration. If an average of severalintervals is used to estimate the heart rate of the implantee, a longinterval caused by underdetection may throw several calculations intoquestion. For at least this reason, the method of FIG. 9 may incorporatea fast tachy detection profile 1070 shown in FIG. 12.

FIG. 11 illustrates a set of detection profile configurations for anillustrative example. The detection profiles include a slow profilehaving similar and dissimilar variants 400, 410, and a fast profile 420with similar and dissimilar variants. The variants on the fast profile420 are shown together to simplify the illustration. In FIG. 11, theprofiles are drawn to scale to show the differences in duration and inrelative scaling of amplitudes. Refractory periods are shown withcross-hatching and have heights that correspond to an estimated peak.

In an illustrative example, “slow” means less than about 147 bpm, “fast”means greater than about 167 bpm, and a hysteresis band is used inbetween, in fashion similar to that explained above with reference toFIG. 9. In other examples, the hysteresis band may be larger, smaller,or omitted. The upper bound of “slow” may be anywhere in the range of100-200 bpm, and the lower bound of “fast” may be in the range of, forexample, 120-240 bpm, for example. These values may be modified further,if desired.

The illustrative slow similar profile 400 is for use when the calculatedheart rate for the implantee is relatively slow and the peak amplitudesof a selected pair of detected events are similar to one another. Theillustrative example uses a 200 millisecond refractory period, followedby a 200 millisecond first constant-threshold period at an amplitude of80% of the estimated peak, followed by a 4 millisecond second constantthreshold period at an amplitude of 50% of the estimated peak, followedby a first time-decaying portion starting at an amplitude of 50% of theestimated peak and decaying to 37.5% of the estimated peak using a timeconstant of 400 milliseconds. The first time-decaying portion of theillustrative slow similar profile 400 ends 720 milliseconds from thestart of the refractory period and is followed by a second time-decayingportion that starts with an amplitude of 37.5% of the estimated peak anddecays to the detection floor using a 400 millisecond time constant.

The illustrative slow dissimilar profile 410 is for use when thecalculated rate for the implantee is relatively slow and the peakamplitudes of a selected pair of detected events are dissimilar from oneanother. The illustrative example uses a 200 millisecond refractoryperiod, followed by a 350 millisecond first constant threshold period atan amplitude of 95% of the estimated peak, followed by a second constantthreshold period having a duration of 4 milliseconds at an amplitude of50% of the estimated peak. The first “decay” period is actually used asa continuation of the second constant threshold period, as there is nodecay since the threshold remains at an amplitude of 50% of theestimated peak until expiration of the first decay period, which occurs720 milliseconds after the beginning of the refractory period. A secondtime-decaying portion follows, beginning at an amplitude of 50% of theestimated peak and decaying to the detection floor using a 400millisecond time constant.

The illustrative fast profile 420 is for use when the calculated rate inthe implantee is relatively fast. For efficient illustration, both thesimilar and dissimilar profiles are shown at 420. The fast profile 420,in the illustrative example, includes a 156 millisecond refractoryperiod followed by a first constant threshold portion having duration of80 milliseconds and an amplitude of 60% of the estimated peak. The firstconstant threshold period is followed by a second constant thresholdperiod having duration of 4 milliseconds with amplitude that varies inresponse to similarity/dissimilarity. A dynamic floor is also defined atthe same amplitude as the second constant threshold period, such thatthe first “decay” time period actually does not decay.

When similar peaks are identified, the fast profile 420 uses 37.5% ofthe estimated peak for the second constant threshold period and thedynamic floor. When dissimilar peaks are identified, the fast profile420 uses 50% of the estimated peak for the second constant thresholdperiod and the dynamic floor.

The profiles are summarized here:

Slow Dissim Slow Similar Fast Dissim Fast Similar Ref (ms) 200 200 156156 CT1 (%) 95 80 60 60 CT1 (ms) 350 200 80 80 CT2 (%) 50 50 50 37.5 CT2(ms) 4 4 4 4 DF (%) 50 37.5 50 37.5 DFTO (ms) 720 720 720 720 TimeConstant 400 400 220 220 (ms)

These values are merely illustrative of one embodiment, and may vary. Inshort, the method selects between a first pair of detection profileswhen rates are relatively low, using peak similarity/dissimilarity todetermine which profile to use. Further in the illustrative example, themethod selects between a second pair of profiles when rates arerelatively high, again using peak similarity/dissimilarity to determinewhich profile to use.

It can be seen in the illustrative example that the fast profiles 420are more sensitive than the slow profiles, and the similar profiles aremore sensitive than the dissimilar profiles. The greater sensitivity ofthe fast profiles 420 may help to track a malignant fast arrhythmia torelatively low amplitudes. This allows the detection profile to matchthe often low amplitude of malignant fast arrhythmia such as VFrelatively quickly.

To illustrate, if an overdetection identification method uses patternidentification to determine that overdetection is occurring, detectionprofile manipulation that prevents some, but not all, overdetection mayimpede the pattern identification. Increasing sensitivity at high ratesmay avoid interference between the two system tools.

FIG. 12 illustrates a full set of detection profile configurations foranother, detailed illustrative example. The level of detail in theexample is not intended to limit the invention to any particular set ofprofiles and/or level of complexity. The illustrative example of FIG. 12integrates several concepts including the use of multiple profiles,definition of fast and slow profiles, and the use of a tachyarrhythmiacondition. Before explaining each profile, the sensing parametersincluding each of Tachy On/Off, and fast/slow are defined.

Tachy On/Off:

In the illustrative example, a tachycardia zone is defined for animplantable device as a programmable parameter. In particular, aphysician or other user of the programmer 44 (FIG. 2) can set the lowestrate for which a tachycardia will be declared. Rates are showngraphically at 1000, with VT Zone PP representing the tachycardia zoneprogrammable parameter. In the illustrative example, VT Zone PP can beset in a range of 170 bpm to 240 bpm. Any time the calculated rate forthe illustrative example exceeds the VT Zone PP, a tachycardia conditionis invoked.

Once a tachycardia condition is invoked, the device enters a “Tachy On”condition. The “Tachy On” condition remains in effect until thecondition terminates. In an illustrative example, the Tachy On conditionis terminated once a predetermined number of consecutive events arecaptured at a rate below the VT Zone PP rate. In a working embodiment,24 consecutive rate calculations below VT Zone PP will terminate theTachy On condition. An offset to the VT Zone PP may also be used toprevent toggling of Tachy On/Off, in addition to or as a substitute forthe 24 consecutive calculations below VT Zone PP. Any time the “TachyOn” condition is not in effect, the device is in a “Tachy Off”condition.

Fast/Slow:

Next, with respect to definitions for Fast and Slow, a numeric exampleis shown at 1000. Rates below a low threshold are considered slow, andrates above a high threshold are considered fast in the illustrativeembodiment. Rates between the thresholds fall within a hysteresis zone.When in the hysteresis zone, the rate is considered fast if the previousrate calculation was also considered fast, and slow if the previousrates calculation was considered slow. In the example, VT Zone PP isprogrammable to values that are above the high threshold. Therefore,some rates will be considered “Fast” but will not meet the criteria tocreate a “Tachy On” condition. Illustrative values for the high and lowthresholds are shown as 148 and 167 bpm; the invention is not limited tothese values. The 24 consecutive calculation rule used to determine theend of a Tachy On condition means that it is also possible to have aSlow rate while the “Tachy On” condition is still invoked.

Post-Shock Special Case:

Finally, a special case is encompassed by the illustrative example. Inthe illustrative example, data seeding occurs following delivery of astimulus shock. This is disclosed in U.S. patent application Ser. No.12/355,552, published as US Patent Application Publication Number2009-0187227, titled DATA MANIPULATION FOLLOWING DELIVERY OF A CARDIACSTIMULUS IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE. In addition to dataseeding, the dynamic floor may be enabled, without changing the Tachy Oncondition. As a result, following delivery of a stimulus shock, theillustrative example enables the dynamic floor until a beat rate aboveVT Zone PP is calculated. As a result, post-shock sensing includes aspecial state in of Tachy On, Dynamic Floor On, referred to as thePost-Shock Tachy On condition.

With the above conditions set forth, the seven profiles shown in FIG. 12can be explained as follows:

-   -   Detection profile 1010 is for use in the Tachy Off condition, as        well as in the Post-Shock Dynamic Floor On condition when        detected events display similar amplitudes.    -   Detection profile 1020 is for use in the Tachy Off condition, as        well as in the Post-Shock Dynamic Floor On condition when        detected events display dissimilar amplitudes.    -   Detection profile 1030 is for use in the Tachy Off condition        where rates are Fast and detected events display similar        amplitudes.    -   Detection profile 1040 is for use in the Tachy Off condition        where rates are Fast and detected events display dissimilar        amplitudes.    -   Detection profile 1050 is for use in the Tachy On condition        where rates are slow and detected events display similar        amplitudes.    -   Detection profile 1060 is for use in the Tachy On condition        where rates are slow and detected events display dissimilar        amplitudes.    -   Detection profile 1070 is for use in the Tachy On condition with        fast rate.

As shown, the use of a dynamic floor in these detection profiles dependson whether a Tachy Off condition is occurring, except for the post-shockspecial case. As a result, profiles 1050, 1060 and 1070 do not show adynamic floor. Instead, the first decay period is shown as decaying tothe noise floor or sensing floor of the system.

As can be seen, the system allows for a large number of differentvariables to be manipulated. The following table provides numericinformation for the illustrative example, with amplitudes provided as apercentage of estimated peak, and durations provided in milliseconds:

Profile 1010 1020 1030 1040 1050 1060 1070 Ref (ms) 200 200 156 156 200200 156 CT1 (ms) 200 350 80 80 200 350 80 CT1 (%) 80 95 60 60 80 95 60CT2 (%) 50 50 37.5 50 50 50 37.5 DF (%) 37.5 50 37.5 50 * * * DFTO (ms)720 720 720 720 * * *

FIG. 12 is intended to be an illustrative example, and the particularconfigurations, features and numeric examples shown are not intended tolimit the present invention.

For any embodiment herein that makes reference to a decay period, anysuitable shape may be used. In some examples, this may includeexponential decay, any other asymptotic decay, or straight-line decay.Also, while the above embodiments refer to constant threshold periods,substituting a decay period is encompassed in additional embodiments.Ramping of the profile by increasing the threshold during a time periodis another alternative that may replace decay or constant thresholdperiods.

While continuous or analog signals are shown in the illustrativeexamples, those of skill in the art will recognize the detection profileand/or captured signal may also be represented in the digital domain,such that a digital approximation of any of these decays is implemented.

As noted above, implantable devices typically use heart rate eitheralone or in conjunction with some other factor to determine whether theimplantee needs therapy. “Some other factor” may include any suitablefactor such as, for example, the morphology/shape of cardiac signalsassociated with detected events, and/or observation of any non-cardiacand/or non-electrical signal. An example of morphology analysis includescorrelation analysis relative to a stored template representing apredetermined cardiac condition, such as a normal sinus rhythm or somepredetermined arrhythmic condition such as atrial fibrillation.Difference of area and difference of squares are two forms ofcorrelation analysis that may be performed. Other analysis, such asprinciple components analysis, source separation, wavelet transform andother mathematical analytics could also be performed as part ofmorphology analysis.

Some illustrative non-cardiac or non-electrical signals may include, forexample, pulse oximetry data, patient respiration data, accelerometerdata indicating patient movement, optical interrogation of bloodcomposition, or any other suitable factor including measured temperatureor blood pressure within an implantee. Some of these factors may becalculated using tissue impedance measurements. Non-cardiac signals maybe used in several forms including, for example, to ensure that acaptured electric signal is in fact a cardiac signal, or to informdecision making by providing an indication of patient status (forexample, Is the patient's breathing accelerated, labored, normal, orstopped?, or Is the patient upright or laying down?). The presentinvention contemplates embodiments in which these additional factors, orany other suitable factor, are included in making stimulus deliverydecisions.

The formula provided above for determining whether “similar” or“dissimilar” events are occurring is an illustrative example. Theapproach shown compares the two most recent peaks to determine whetherthey are similar. Other factors may be used. For example, a system maymaintain statistics regarding prior peak activity or trend activity andmay use the average or trend average and a standard deviation orvariance to determine whether a newly detected event likely falls within“similar” or “dissimilar” bounds.

In yet another example, similar/dissimilar may be determined relative tothe estimated peak, rather than the most recent peak. In anotherexample, peak-to-peak ratios are calculated and recorded to generatestatistics for peak ratios. An unexpected peak ratio outcome fallingoutside of statistical bounds may be considered as indicatingdissimilarity.

Hysteresis may be built into the similar/dissimilar identification step.For example, a three part range for the peak ratio may be used asfollows:

Range Outcome Peak Ratio > 1.3 or Peak Ratio < 0.7 Dissimilar 0.9 < PeakRatio < 1.1 Similar Else Same as Prior OutcomeIn this example, a hysteresis band is built into the peak ratiocalculation.

Peak similarity is one method of determining whether consecutivedetected events are similar or dissimilar. Another method may includemorphological analysis. For example, two consecutive events may beanalyzed by correlation waveform analysis to determine whether the twoevents are similar or dissimilar. In another example, a series ofdetected events may each be compared to a template to determine whethersimilarity or dissimilarity relative to the template occurs. In yetanother example, rather than comparing two events to one another, eventsmay be compared in a string of comparisons, for example, Event(n) may becompared to each of Event(n−1) and Event(n−2) to observe whethersimilar/dissimilar patterns emerge, likely indicating overcounting and,in the illustrative example, justifying the use of a less sensitivedetection profile.

Given the nature of the comparisons taking place, it is also accurate todescribe the comparison of a detected event to a previous detectedevent, either in simple amplitude or in morphology, as comparison of thedetected event to stored data to determine the similarity of a mostrecent detected event to the stored data. The stored data may come fromanalysis of one or more prior events. This provides a more genericdescription of the underlying activity.

As noted above, other measures of estimated peak may also be used. Theabove examples simply average two prior peak amplitudes. The followingare additional illustrative Estimated Peak calculations:Est Peak[n]=Peak[n−1]Est Peak[n]=(Peak[n−1]+Peak[n−2])/2Est Peak[n]=(Peak[n−1]+Est Peak[n−1])/2Where [n] represents the event under consideration, and [n−1, n−2]represent prior detected events. Other, more complex functions may beused. In another embodiment, the similarity/dissimilarity of a newlydetected peak to the prior peak or estimated peak may be analyzed todetermine whether to exclude the new detected peak from an updatedcalculation of estimated peak.

As noted above, various changes to the values provided, for example,with reference to FIGS. 7A-7B, the following ranges are illustrative:

Dissimilar Similar Refractory: 50-350 ms  50-250 ms  CT1%:  80-110%60-85% CT1 Duration: 0-400 ms 0-300 ms CT2%: 40-90% 30-60% CT2 Duration:0-200 ms 0-200 ms DF %: 30-70% 25-50% DF TO: 500-1500 ms from start ofRefractory

Further, as discussed above, in addition to comparing the peaksimilarity or other characteristic of consecutive detections, the periodbetween consecutive detections may also control which detection profileis invoked. In on example, if the period between two detections exceedsa threshold of, for example, 500-1000 ms, it is assumed that thedetections do not originate in a single cardiac cycle, and a “Similar”detection profile is invoked.

Following are certain additional configuration examples:

Example A

Dissimilar Similar Refractory: 150 ms 150 ms CT1%: 90% 80% CT1 Duration:200 ms 200 ms CT2%: 75% 60% CT2 Duration: 300 ms  20 ms DF %: 45% 45% DFTO: 800 ms from start of Refractory

Example B

Dissimilar Similar Refractory: 100 ms 200 ms CT1 Amplitude: 80% 80% CT1Duration: 200 ms 200 ms CT2 Amplitude: — 50% CT2 Duration: — 100 ms DF%: 35% 35% DTO: 1250 ms from start of Refractory

Note in Example B, the CT2 component is excluded from the detectionprofile when dissimilar events are identified. Some embodimentsincorporate this variation. In addition, the Dissimilar Profile is moresensitive here than the Similar profile, by virtue of a shorterrefractory period and omission of the CT2 parameters. As noted above,this may encourage consistent overdetection that can be identified andcorrected by other methods.

In some examples, the above configurations are modified in certain waysto incorporate the following form:Threshold Amplitude=P% of Est Peak+Constant

For example:

Dissimilar Similar Refractory: 200 ms 200 ms CT1%: 80% + 25ADC 80% CT1Duration: 350 ms 200 ms CT2%: 50% + 25ADC 50% CT2 Duration: 200 ms 100ms

In this example, “25ADC” means twenty-five ADC units. Within thisillustrative configuration, a maximum value for CT1% and CT2% may be setto the maximum dynamic range of the ADC output, or to some otherpredetermined maximum.

The above illustrative examples may be embodied in many suitable forms.Some embodiments will be method embodiments incorporating one or more ofthe above features/sub-methods in various combinations. Some embodimentswill be devices adapted to perform the methods discussed above. Someembodiments will take the form of tangible media, such as magnetic,electric, or optical storage media, incorporating controller readableinstruction sets. Some embodiments will take the form of or comprisecontrollers/microcontrollers associated with stored instruction sets fordirecting operations of various components in a device in accordancewith one or more methods.

Briefly, an illustrative example may make use of amicrocontroller-driven system which includes an input switch matrix forselecting one or more signal vectors as a sensing vector. The switchmatrix leads to one or more amplifiers and filtering circuits that inturn couple to analog-to-digital conversion circuitry. Additionalfiltering of the incoming signal may be performed in the digital domainincluding, for example, 50/60 Hz notch filters. The incoming signal maythen be analyzed using the microcontroller and any associated suitableregisters and logic circuits. Some embodiments include, for example,dedicated hardware for peak or event detection and measurement, or forcorrelation waveform analysis.

In several illustrative examples, upon identification of a rhythm thatindicates stimulus, a charging operation is undertaken. A sub-circuitfor charging high-voltage or stimulus capacitors may have any suitableform. One example uses a charger taking the form of a flybacktransformer circuit, a structure well known in the art. Any processand/or circuit that enables relatively low voltage batteries to chargecapacitors to relatively high voltages may be used.

The device may further include output circuitry comprising, for example,an output H-bridge or modification thereof for controlling outputpolarity and duration from the high-power capacitor. Control circuitryassociated with the H-bridge may be included, for example, to monitor orcontrol current levels for constant current output signals or forperforming diagnostic functions. The circuitry may be housed in ahermetically sealed canister.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departures in form anddetail may be made without departing from the scope and spirit of thepresent invention.

What is claimed is:
 1. A method of event detection in an implantablecardiac stimulus device having a plurality of electrodes adapted forimplantation in a patient, the method comprising: capturing electricalsignals from the electrodes; comparing captured electrical signals to adetection threshold that varies with time following a detection profile;when the captured electrical signals cross the detection threshold,declaring a detected event; determining whether either of theseconditions is met: a) the detected event is similar to a prior detectedevent, or b) whether the detected event is separated from an immediatelyprior detected event by an interval exceeding an interval threshold;selecting from between first and second variants of a selected detectionprofile for use in subsequent event detection as follows: if b) is met,then selecting the first variant; if b) is not met and a) is met, thenselecting the first variant; and else selecting the second variant; anddetecting a subsequent cardiac event using the selected variant of theselected detection profile.
 2. The method of claim 1 wherein theselected detection profile is selected by selecting from among aplurality of detection profiles each corresponding to one of: acalculated beat rate range for a patient; and a post-shock state of thedevice.
 3. The method of claim 1 wherein each of the first and secondvariants are dependent upon an amplitude measure of the prior detectedevent.
 4. The method of claim 3 wherein the first and second variantseach include a period in which the detection profile is flat for a fixedperiod and is calculated as a fixed percentage of the amplitude measureof the prior detected event such that: the first variant has a firstfixed period and a first fixed percentage, and the second variant has asecond fixed period that is longer than the first fixed period, and asecond fixed amplitude that is higher than the first fixed amplitude. 5.An implantable cardiac stimulus device having a plurality of electrodesadapted for implantation in a patient and operational circuitry coupledto the plurality of electrodes which is configured to perform a methodcomprising: capturing electrical signals from the electrodes; comparingcaptured electrical signals to a detection threshold that varies withtime following a detection profile; when the captured electrical signalscross the detection threshold, declaring a detected event; determiningwhether either of these conditions is met: a) the detected event issimilar to a prior detected event, or b) whether the detected event isseparated from an immediately prior detected event by an intervalexceeding an interval threshold; selecting from between first and secondvariants of a selected detection profile for use in subsequent eventdetection as follows: if b) is met, then selecting the first variant; ifb) is not met and a) is met, then selecting the first variant; and elseselecting the second variant; and detecting a subsequent cardiac eventusing the selected variant of the selected detection profile.
 6. Thedevice of claim 5 wherein the operational circuitry is configured suchthat the selected detection profile is selected by selecting from amonga plurality of detection profiles each corresponding to one of: acalculated beat rate range for a patient; and a post-shock state of thedevice.
 7. The device of claim 5 wherein the operational circuitry isconfigured so that each of the first and second variants are dependentupon an amplitude measure of the prior detected event.
 8. The device ofclaim 7 wherein the operational circuitry is configured so that thefirst and second variants each include a period in which the detectionprofile is flat for a fixed period and is calculated as a fixedpercentage of the amplitude measure of the prior detected event suchthat: the first variant has a first fixed period and a first fixedpercentage, and the second variant has a second fixed period that islonger than the first fixed period, and a second fixed amplitude that ishigher than the first fixed amplitude.
 9. An implantable cardiacstimulus device having a plurality of electrodes adapted forimplantation in a patient and means for analyzing signals captured bythe plurality of electrodes, wherein the means for analyzing isconfigured to perform a method comprising: comparing captured electricalsignals from the plurality of electrodes to a detection threshold thatvaries with time following a detection profile; when the capturedelectrical signals cross the detection threshold, declaring a detectedevent; determining whether either of these conditions is met: a) thedetected event is similar to a prior detected event, or b) whether thedetected event is separated from an immediately prior detected event byan interval exceeding an interval threshold; selecting from betweenfirst and second variants of a selected detection profile for use insubsequent event detection as follows: if b) is met, then selecting thefirst variant; if b) is not met and a) is met, then selecting the firstvariant; and else selecting the second variant; and detecting asubsequent cardiac event using the selected variant of the selecteddetection profile.
 10. The device of claim 9 wherein means for analyzingis configured such that the selected detection profile is selected byselecting from among a plurality of detection profiles eachcorresponding to one of: a calculated beat rate range for a patient; anda post-shock state of the device.
 11. The device of claim 9 wherein themeans for analyzing is configured such that each of the first and secondvariants are dependent upon an amplitude measure of the prior detectedevent.
 12. The device of claim 11 wherein the means for analyzing isconfigured such that the first and second variants each include a periodin which the detection profile is flat for a fixed period and iscalculated as a fixed percentage of the amplitude measure of the priordetected event such that: the first variant has a first fixed period anda first fixed percentage, and the second variant has a second fixedperiod that is longer than the first fixed period, and a second fixedamplitude that is higher than the first fixed amplitude.